Fig. 2.

An increasing global mortality rate is predicted to favor the fast grower. a, b Here we illustrate the parameters of the Lotka–Volterra (LV) interspecific competition model with added mortality: population density N, growth r, death δ, and the strengths of inhibition α_sf and α_fs (subscript f for fast grower and s for slow grower). Here we assume a continuous death rate, but in the model, the outcome is the same for a discrete process, such as our daily dilution factor (Supplementary Note 2). The width of arrows in a corresponds to an interesting case that we observe experimentally, in which the fast grower is a relatively weak competitor. c The outcomes of the LV model without mortality depend solely upon the competition coefficients α, and the phase space is divided into one quadrant per outcome. If the slow grower is a strong competitor, it can exclude the fast grower. Imposing a uniform mortality rate δ on the system, however, favors the faster grower by making the re-parameterized competition coefficients $\tilde{\alpha }$ depend on r and δ. Given that a slow grower dominates at low or no added death, the model predicts that coexistence or bistability will occur at intermediate added death rates before the outcome transitions to dominance of the fast grower at high added death (Supplementary Note 1). Two numerical examples show that the values of α (in the absence of added mortality) determine whether the trajectory crosses the bistability or coexistence region as mortality increases